Images captured by an imaging device, such as a digital camera or a scanner, contain noise, such as shot noise or dark current noise, due to the characteristics of an imaging element and a circuit. To obtain high-quality images from the captured images, it is necessary to reduce noise in the images. However, if a low-pass filter is simply used to reduce noise, elements, such as edges, that are important in human perception of the images are also lost while reducing the noise, resulting in low image quality. Therefore, it is necessary to adaptively reduce noise according to the characteristics of image regions.
As noise reduction technologies, an s-filter and a bilateral filter have been developed (see Non-Patent Document 1 (Hiroshi Harashima, Kaoru Odajima, Yoshiaki Shishikui, Hiroshi Miyakawa “ε-Separating Nonlinear Digital Filter and Its Applications”, IEICE Transactions A, Vol. J65-A, No. 4, pp. 297-304, 1982) and Non-Patent Document 2 (C C. Tomasi and R. Manduchi, “Bilateral Filtering for Gray and Color Images” Proc. Sixth Int'l Conf. Computer Vision, pp. 839-846, 1998)). With the ε-filter, if filtering is performed based on neighboring pixels at which signal differences from a pixel of interest are equal to or smaller than a predetermined threshold, noise can be reduced while components, such as edges, having greater signal differences can be preserved (see Non-Patent Document 1).
Similarly to the ε-filter, the bilateral filter can enable edge preserving and noise reduction at the same time (see Non-Patent Document 2). The bilateral filter is a technique for performing filtering according to a weight coefficient that is generated based on a signal difference and a spatial distance from a pixel of interest. Other noise reductions based on the principle equivalent to the bilateral filter have also been proposed (see, for example, Patent Document 1 (Japanese Patent Application Laid-open No. 2007-288439), Patent Document 2 (Japanese Patent Application Laid-open No. 2008-205737), Patent Document 3 (Japanese Patent Application Laid-open No. 2010-178302), and Patent Document 4 (Japanese Patent Application Laid-open No. 2010-087769)).
Furthermore, Patent Document 5 (Japanese Patent Application Laid-open No. 2010-218110) discloses a technology for reducing signal-dependent noise in color images without performing iterative processing, such as segmentation. This technology is based on the principle equivalent to the ε-filter. Specifically, a similar pixel group around a pixel of interest is selected and the characteristics of the similar pixel group are reflected in a conversion process to convert a difference between a result obtained by operation using the ε-filter and a pixel value to be processed.
However, the conventional noise reduction process has a problem with an increase in costs. For example, in the process disclosed in Patent Document 5, it is necessary to perform filtering operation at all of the pixel positions in an image, so that a calculation cost can hardly be reduced. Furthermore, Non-Patent Document 3 (Ce Liu, et al., “Automatic Estimation and Removal of Noise from a Single Image,” IEEE Trans. Pattern Analysis and Machine Intelligence, vol. 30, no. 2, pp. 299-314, 2008) discloses a technology for reducing signal-dependent noise in a color image. However, in this method, large processing costs are needed for image segmentation or statistics calculation.
Therefore, there is a need to provide an image processing apparatus, an imaging device, an image processing method, and a computer-readable recording medium capable of reducing a processing cost in a noise reduction process on images.